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| Michelle Liu | michelcl@usc.edu | Undeclared Engineering |
Nuclear Energy: Overview
Nuclear power is about 20 percent of the total electricity generated in USA . The process that produces the heat in a nuclear plant is the fissioning or splitting of uranium atoms.
“NUCLEAR REACTOR” is an apparatus in which nuclear fission chain reactions are initiated, controlled, and sustained at a contained rate. Although the majority of nuclear ractors are gnerating electricity, some are used for production of radioactive isotopes for medical and industrial use, or the production of plutonium for nuclear weapons.
CERRENT FAMILIES OF REACTORS are PWR(Pressurized water reactor), BWR (Boiling water reactor), PHWR(Pressurised Heavy Water Reactor, AGR(Advanced gas-cooled Reactor), RBMK(Light water cooled graphite moderated reactor), and D2G reactor.
The two types of reactors, mostly used in USA , are boiling-water reactors(BWRs), and pressurized-water reactors(PWRs). Roughly 70 percent of the reactors operating in U.S. are PWR. In a PWR, the water passing through the reactor core is kept under pressure so that it does not turn to steam at all (it remains liquid). In the BWR, the water instead of remaining liquid, it is heated by the reactor core turns directly into steam in the reactor vessel and is then used to power the turbine-generator. Nuclear reactors are basically machines that contain and control chain reactions, while releasing heat at a controlled rate. In electric power plants, the reactors supply the heat to turn water into steam, which drives the turbine-generators.
The reactor core is composed of 4 main elements: THE FUEL, THE CONTROL RODS, THE COOLANT, and THE MODERATOR.
THE FUEL: Nuclear fuel consists of pellets of enriched uranium dioxide encased in 12-foot long pencil-thick metal tubes, called fuel rods. A nuclear plant can operate continuously for up to 2 years, and the ractor must have 100 to 300 fuel assemblies.
THE CONTROL RODS: contain material that regulates the rate of the chain reaction.
THE COOLANT: usually “WATER”. This system can be compared to the cooling system of a car, which can carry away the heat built up in the engine. In a reactor, in order to carry away the heat, it needs 330,000 gallons of water flow through the reactor core every minute.
THE MODERATOR: “WATER”. It slows down the speed at which atoms travel, and the reduction in speed increases the opportunity to split, thereby releasing energy.
The technology of nuclear power emits no airborn pollutants, and overall far less waste material than fossil fuel based power plants, so nuclear generated electricity does not emit carbon dioxide into the atmosphere, which can cause several problems, like greenhouse effect. Also, nuclear power have some useful additional advantages such as the production of radioisotopes.
The nuclear power generation also have some disadvantages: Low dose radiation released under normal operating conditions or during waste spills is also a concern. The use of nuclear reactors is the threat of an accident or terrorist attack and resulting exposure to radiation, however, even in an accident such as Three Mile Island, the containment vessels were never breached, so that very little radiation was exposed to environment.Spent nuclear fuels are radioactive, they are pound for pound a more substantial problem, and the irradiated fuel assemblies are highly radioactive and must be stored in specially designed pools resembling large swimming pools (water cools the fuel and acts as a radiation shield) or in specially designed dry storage containers. The U.S. Department of Energy’s long range plan is for this spent fuel to be stored deep in the earth in a geologic repository, at Yucca Mountain , Nevada . Currently all spent ( used) fuel is stored at the power plant at which it was used.
In France , 80% of all elctric power comes from nuclear ractors.
In 2000, throughout the world, there were 438 commercial nuclear generating units with a capacity of about 351 gigawatts.
In 2001, there were 104 (69 pressurized water reactors, 35 boiling water reactors) commercial nuclear generating units that are licensed to operate in U.S, producing 32,300 net megawatts(electric), and they were located mostly on the East Coast and in the Midwest.